<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>

  </style>
</head>
<body>
  <div id="app">

  </div>

  <script>
      class MyButton{
        btn = null;

        constructor(){
          this.created();

          this.mounted();
        }

        created() {
          this.btn = document.createElement('button');
          this.btn.innerHTML = 'click me';
          this.btn.addEventListener('click', function(){
            console.log('haha');
          }, false);
        }

        mounted() {
          let app = document.getElementById('app');
          app.appendChild(this.btn);
        }
      }

      new MyButton();
  </script>
</body>
</html>